The ejector seat manufacturer Martin-Baker Aircraft has admitted breaching health and safety law over the death of a Red Arrows pilot.

Flt Lt Sean Cunningham died in November 2011 at RAF Scampton in Lincolnshire after his ejector seat initiated during the pre-flight checks of his Hawk T1 jet while on the ground and stationary.

The parachute on the ejector seat did not deploy and the South African-born airman later died in hospital.

The statutory director of Martin-Baker Aircraft, John Martin, pleaded guilty on behalf of the company at Lincoln crown court on Monday to a charge under section 3(1) of the Health and Safety at Work Act 1974.

The prosecutor Rex Tedd told the court: “The first matter in dispute is the question of design. We say it was defective from the outset.”

An inquest into Cunningham’s death in 2014 heard that the ejector seat firing handle had been left in an unsafe position, meaning it could accidentally activate the seat.

Stuart Fisher, the senior coroner for Lincolnshire, criticised Martin-Baker for failing to warn the RAF about safety issues.

Fisher recorded a narrative verdict, describing a safety pin that went through the firing handle as “entirely useless” and concluding that its presence was “likely to mislead”.

The coroner said that despite 19 checks carried out on the Hawk T1, there was a repeated failure to notice that the safety pin had been incorrectly housed and that the seat firing handle was in an unsafe position.

He said tests showed the pin could be inserted into the MK10 ejection seat even when it was in an unsafe position, giving the impression to RAF personnel that the seat was safe.

Despite Martin-Baker being aware since 1990 of issues with the over-tightening of crucial nuts and bolts in the mechanism of the seat that would cause the main parachute not to deploy properly, the company failed to pass on the warnings to the Ministry of Defence, Fisher said.

A statement from Martin-Baker offered condolences to Cunningham’s family and friends. The company said it had entered a guilty plea after the Health and Safety Executive “considerably narrowed” the issues from when its investigation first started.

“It should be noted that this was an isolated failure relating to the tightening of a nut during maintenance procedures conducted by RAF Aerobatic Team mechanics,” the statement added.